The present invention relates generally to a frequency encoding circuit for a Bragg cell receiver (or other types of channelized receivers) to reduce false alarms. More particularly, the invention relates to the field of receivers which use acousto-optic delay lines to analyze signals such as radar pulses present in the Electronic Warfare (EW) environment. The circuit may be used to encode the channel outputs of a Bragg cell receiver, or other types of channelized receiver. The receivers are of the type proposed for use in Electronic Warfare requiring high probability of intercept with low false alarm rates.
The present design in determining frequency from a Bragg receiver is by comparing the amplitude from adjacent outputs as shown in FIG. 2. If the output of a certain channel is higher than its neighboring ones and the signal crosses threshold, the frequency of the input signal is determined by that channel. There is one major deficiency from this approach, that is, the design generates false alarms. For example, in FIG. 2, if the input signal is in channel C, the output from C is much stronger than B and D. As a result, the output from C is one, the remaining outputs are zeros, and there are no false alarms. However, if the input signal is in H, the signal strengths in B, C, and D are weak. Theoretically, the outputs should be D&gt;C&gt;B, because D is closer to the signal than C which is closer than B. The difference may be very small, because they are far away from the input signal. If the gain in every channel is not perfectly balanced, i.e., in actual hardware design, it is possible that D&lt;C&gt;B. Under this condition, the output C does have an output which represents a false alarm. This phenomenon occurs especially in receivers designed with high dynamic range.
The following United States patents are of interest.
U.S. Pat. No. 4,328,576--Oakley
U.S. Pat. No. 4,644,267--Tsui et al
The Oakley patent describes a system in which phase shift keyed signals are detected and demodulated over a wide bandwidth using a two-channel Bragg cell having two sonic input transducers.
The Tsui et al patent describes a system which makes use on an interferometric Bragg cell for the time delays needed by multiple antenna inputs in an angle-of-arrival (AOA) measurement system, for accurate measurements over wide bandwidths.
A paper by E. T. Gill and J. B. Y. Tsui on "Interferometric Acousto-optic Receiver Results" reports experimental results and conclusions on the performance of an interferometric acousto-optic receiver (IAOR). The receiver is designed to acousto-optically channelize a 1 GHz RF bandwidth spectrum into 128 frequency bins, and generate a digital frequency, pulse amplitude, pulse width and TOA (time-of-arrival) words. This paper and the Tsui patent are hereby incorporated by reference.